do not add MenuButton to ButtonBox

ButtonBox only accepts PushButtons, trying to add a MenuButton results
in crash. Instead, put the ButtonBox and the MenuButton in a HBox.
See bnc#881798 for details.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Add change log.
Merge pull request #17 from varkoly/master

Do not add MenuButton to ButtonBox